The present invention is particularly directed to vitrified bonded finishing and superfinishing sticks (also called honing sticks) which are prepared from ultrafine grain sol gel aluminous abrasive grit. Vitrified bonded abrasive products are different from resinoid bonded, i.e. phenol-aldehyde, products in that they use a glass phase to bond the grit and thus are processed at substantially higher temperatures (about 800.degree. C. or higher for vitrified vs. about 400.degree. C. or lower for resinoid). Due to the higher manufacturing temperature used, vitrified bonded products can withstand higher temperatures in use.
Attempts at preparing vitrified bonded products from ultrafine sol gel sintered alumina abrasive grits, especially those having an average particle size less than about 20.mu., have largely been unsuccessful. Such ultrafine particles have a much higher surface reactivity than do both coarser sol-gel aluminous particles and conventional fused alumina particles. The surface reactivity has been found to be a particular problem when attempting to use such particles with vitrified bonds which are processed at elevated temperature. As a result, only very soft, poor performing products have been produced heretofore from the ultrafine particles. In fact, the products have generally been inferior to similar products prepared from conventional fused alumina. Since relatively large particle size sol-gel aluminous abrasive grits have generally demonstrated advantages over other alumina abrasive materials, there has been a desire to develop a procedure for incorporating ultrafine grits into vitrified bonds in an attempt to obtain similar substantial performance advantages. Sol-gel abrasives are generally made by forming a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2 and then drying and sintering the gel. See for example, U.S. Pat. No. 4,314,827. Seeded sol-gel abrasives such as those formed by the conversion of hydrated alumina to alpha alumina by the use of alpha alumina seed or functional equivalent materials having the same crystal structure and similar lattice parameters, such as ferric oxide and the like. Processes for the production of seeded sol-gel alumina materials are described, for example, in U.S. Pat. Nos. 4,623,364, 4,744,802, 4,954,462, 4,964,883, and PCT Applications WO 90/08744 and WO 90/02160. To produce abrasive material from a gel, the dried and sintered extruded gel material is usually crushed or broken by suitable means such as a hammer or ball mill to form abrasive particles which are generally then size classified. The same type of extrusion, grinding, and forming processes may be used as is done with other abrasive forming compositions than sol-gel materials.
For purposes of this application and the invention disclosed, the term "sol gel sintered alumina abrasive" is intended to apply to abrasives produced by the teachings of any of U.S. Pat. Nos. 4,518,397, 4,623,364, 4,744,802, 4,954,462, 4,964,883, and PCT Applications WO 90/08744 and WO 90/02160 as well as by other sol-gel techniques which accomplish a similar result.
U.S. Patent No. 4,997,461 discloses the use of silica-enriched surfaces on sol-gel aluminous grit to reduce deleterious interaction between a vitrified bond and said grit. It has now been discovered that when the grit particles are very fine, i.e. less than about 20 .mu. silica sources are such good adhesives that extensive agglomeration results during the drying of the silica-coated high surface area fine grits. The agglomerates act as coarse grains in abrasive bodies and defeat the purpose of using the fine grit particles. Silicon is a non-metal.
Accordingly, there is a need for a system which is useful particularly with very fine sol-gel aluminous grit to form vitrified bodies.